Substituted triazolo-pyridazine derivatives

ABSTRACT

This invention relates to novel substituted triazolo-pyridazines, their derivatives, and pharmaceutically acceptable salts thereof. This invention also provides compositions comprising a compound of this invention and the use of such compositions in methods of treating diseases and conditions that are beneficially treated by administering an α1-GABA-A receptor antagonist and/or a α2, α3 and α5 GABA-A receptor agonist.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/550,346, filed on Aug. 28, 2009, now allowed, which claims thebenefit of priority under 35 USC §119 to U.S. Provisional ApplicationSer. No. 61/093,293, filed on Aug. 29, 2008. The contents of each of theforegoing applications are incorporated herein by reference.

This invention relates to novel substituted triazolo-pyridazines, theirderivatives, and pharmaceutically acceptable salts thereof. Thisinvention also provides compositions comprising a compound of thisinvention and the use of such compositions in methods of treatingdiseases and conditions that are beneficially treated by administeringan α1-GABA-A receptor antagonist.

L-838417, also known as7-tert-butyl-3-(2,5-difluorophenyl)-6-(1-methyl-1H-1,2,4-triazol-5-ylmethoxy)[1,2,4]triazolo[4,3-b]pyridazine,acts at the benzodiazepine site of the GABA-A receptor as an antagonistof α1 subtypes, and as a functionally selective allosteric agonist ofthe α2, α3 and α5 subtypes.

L-838417 is currently a preclinical candidate for central nervous systemdisorders. Despite the beneficial activities of L-838417, there is acontinuing need for new compounds that are α1-GABA-A receptorantagonists.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the stability of compounds of the invention in humanliver microsomes over time.

FIG. 2 depicts the stability of compounds of the invention in rat livermicrosomes over time.

FIG. 3 depicts the change in concentration of compounds of the inventionin plasma following intravenous administration to rats.

FIG. 4 depicts the change in concentration of compounds of the inventionin plasma following oral administration to rats.

DEFINITIONS

The term “treat” means decrease, suppress, attenuate, diminish, arrest,or stabilize the development or progression of a disease (e.g., adisease or disorder delineated herein).

“Disease” means any condition or disorder that damages or interfereswith the normal function of a cell, tissue, or organ.

It will be recognized that some variation of natural isotopic abundanceoccurs in a synthesized compound depending upon the origin of chemicalmaterials used in the synthesis. Thus, a preparation of L-838417 willinherently contain small amounts of deuterated isotopologues. Theconcentration of naturally abundant stable hydrogen and carbon isotopes,notwithstanding this variation, is small and immaterial as compared tothe degree of stable isotopic substitution of compounds of thisinvention. See, for instance, Wada, E et al., Seikagaku, 1994, 66:15;Gannes, L Z et al., Comp Biochem Physiol Mol Integr Physiol, 1998,119:725. In a compound of this invention, when a particular position isdesignated as having deuterium, it is understood that the abundance ofdeuterium at that position is substantially greater than the naturalabundance of deuterium, which is 0.015%. A position designated as havingdeuterium typically has a minimum isotopic enrichment factor of at least3000 (45% deuterium incorporation) at each atom designated as deuteriumin said compound.

The term “isotopic enrichment factor” as used herein means the ratiobetween the isotopic abundance and the natural abundance of a specifiedisotope.

In other embodiments, a compound of this invention has an isotopicenrichment factor for each designated deuterium atom of at least 3500(52.5% deuterium incorporation at each designated deuterium atom), atleast 4000 (60% deuterium incorporation), at least 4500 (67.5% deuteriumincorporation), at least 5000 (75% deuterium), at least 5500 (82.5%deuterium incorporation), at least 6000 (90% deuterium incorporation),at least 6333.3 (95% deuterium incorporation), at least 6466.7 (97%deuterium incorporation), at least 6600 (99% deuterium incorporation),or at least 6633.3 (99.5% deuterium incorporation).

In the compounds of this invention any atom not specifically designatedas a particular isotope is meant to represent any stable isotope of thatatom. Unless otherwise stated, when a position is designatedspecifically as “H” or “hydrogen”, the position is understood to havehydrogen at its natural abundance isotopic composition. Also unlessotherwise stated, when a position is designated specifically as “D” or“deuterium”, the position is understood to have deuterium at anabundance that is at least 3340 times greater than the natural abundanceof deuterium, which is 0.015% (i.e., at least 50.1% incorporation ofdeuterium).

The term “isotopologue” refers to a species that differs from a specificcompound of this invention only in the isotopic composition thereof.

The term “compound,” as used herein, refers to a collection of moleculeshaving an identical chemical structure, except that there may beisotopic variation among the constituent atoms of the molecules. Thus,it will be clear to those of skill in the art that a compoundrepresented by a particular chemical structure containing indicateddeuterium atoms, will also contain lesser amounts of isotopologueshaving hydrogen atoms at one or more of the designated deuteriumpositions in that structure. The relative amount of such isotopologuesin a compound of this invention will depend upon a number of factorsincluding the isotopic purity of deuterated reagents used to make thecompound and the efficiency of incorporation of deuterium in the varioussynthesis steps used to prepare the compound. However, as set forthabove the relative amount of such isotopologues will be less than 49.9%of the compound.

The invention also includes salts of the compounds disclosed herein.

A salt of a compound of this invention is formed between an acid and abasic group of the compound, such as an amino functional group, or abase and an acidic group of the compound, such as a carboxyl functionalgroup. According to another embodiment, the compound is apharmaceutically acceptable acid addition salt.

The term “pharmaceutically acceptable,” as used herein, refers to acomponent that is, within the scope of sound medical judgment, suitablefor use in contact with the tissues of humans and other mammals withoutundue toxicity, irritation, allergic response and the like, and arecommensurate with a reasonable benefit/risk ratio. A “pharmaceuticallyacceptable salt” means any non-toxic salt that, upon administration to arecipient, is capable of providing, either directly or indirectly, acompound of this invention. A “pharmaceutically acceptable counterion”is an ionic portion of a salt that is not toxic when released from thesalt upon administration to a recipient.

Acids commonly employed to form pharmaceutically acceptable saltsinclude inorganic acids such as hydrogen bisulfide, hydrochloric acid,hydrobromic acid, hydroiodic acid, sulfuric acid and phosphoric acid, aswell as organic acids such as para-toluenesulfonic acid, salicylic acid,tartaric acid, bitartaric acid, ascorbic acid, maleic acid, besylicacid, fumaric acid, gluconic acid, glucuronic acid, formic acid,glutamic acid, methanesulfonic acid, ethanesulfonic acid,benzenesulfonic acid, lactic acid, oxalic acid, para-bromophenylsulfonicacid, carbonic acid, succinic acid, citric acid, benzoic acid and aceticacid, as well as related inorganic and organic acids. Suchpharmaceutically acceptable salts thus include sulfate, pyrosulfate,bisulfate, sulfite, bisulfite, phosphate, monohydrogenphosphate,dihydrogenphosphate, metaphosphate, pyrophosphate, chloride, bromide,iodide, acetate, propionate, decanoate, caprylate, acrylate, formate,isobutyrate, caprate, heptanoate, propiolate, oxalate, malonate,succinate, suberate, sebacate, fumarate, maleate, butyne-1,4-dioate,hexyne-1,6-dioate, benzoate, chlorobenzoate, methylbenzoate,dinitrobenzoate, hydroxybenzoate, methoxybenzoate, phthalate,terephthalate, sulfonate, xylene sulfonate, phenylacetate,phenylpropionate, phenylbutyrate, citrate, lactate, β-hydroxybutyrate,glycolate, maleate, tartrate, methanesulfonate, propanesulfonate,naphthalene-1-sulfonate, naphthalene-2-sulfonate, mandelate and othersalts. In one embodiment, pharmaceutically acceptable acid additionsalts include those formed with mineral acids such as hydrochloric acidand hydrobromic acid, and especially those formed with organic acidssuch as maleic acid.

The compounds of the present invention (e.g., compounds of Formula I),may contain an asymmetric carbon atom, for example, as the result ofdeuterium substitution or otherwise. As such, compounds of thisinvention can exist as either individual enantiomers, or mixtures of thetwo enantiomers. Accordingly, a compound of the present invention mayexist as either a racemic mixture or a scalemic mixture, or asindividual respective stereoisomers that are substantially free fromanother possible stereoisomer. The term “substantially free of otherstereoisomers” as used herein means less than 25% of otherstereoisomers, preferably less than 10% of other stereoisomers, morepreferably less than 5% of other stereoisomers and most preferably lessthan 2% of other stereoisomers, or less than “X”% of other stereoisomers(wherein X is a number between 0 and 100, inclusive) are present.Methods of obtaining or synthesizing an individual enantiomer for agiven compound are known in the art and may be applied as practicable tofinal compounds or to starting material or intermediates.

Unless otherwise indicated, when a disclosed compound is named ordepicted by a structure without specifying the stereochemistry and hasone or more chiral centers, it is understood to represent all possiblestereoisomers of the compound.

The term “stable compounds,” as used herein, refers to compounds whichpossess stability sufficient to allow for their manufacture and whichmaintain the integrity of the compound for a sufficient period of timeto be useful for the purposes detailed herein (e.g., formulation intotherapeutic products, intermediates for use in production of therapeuticcompounds, isolatable or storable intermediate compounds, treating adisease or condition responsive to therapeutic agents).

“D” refers to deuterium. “Stereoisomer” refers to both enantiomers anddiastereomers. “Tert”, “^(t)”, and “t-” each refer to tertiary. “US”refers to the United States of America.

Throughout this specification, a variable may be referred to generally(e.g., “each R”) or may be referred to specifically (e.g., R¹, R², R³,etc.). Unless otherwise indicated, when a variable is referred togenerally, it is meant to include all specific embodiments of thatparticular variable.

Therapeutic Compounds

The present invention provides a compound of Formula I:

or a pharmaceutically acceptable salt thereof, wherein:

R¹ is CH₃, CDH₂, CD₂H, or CD₃;

R² is a t-butyl group having 0-9 deuterium atoms;

each Y is independently hydrogen or deuterium; and

when R¹ is CH₃ and each Y is hydrogen, then R² has 1-9 deuterium atoms.

One embodiment of this invention provides a compound of Formula Iwherein R¹ is CH₃ or CD₃. In one aspect of this embodiment, Y^(1a) andY^(1b) are hydrogen. In another aspect, Y^(1a) and Y^(1b) are deuterium.In another aspect, Y² is hydrogen. In another aspect, Y² is deuterium.In another aspect, R² is —C(CH₃)₃ or —C(CD₃)₃. As an example of thisaspect, R² is —C(CD₃)₃.

Another embodiment provides compounds wherein R² is —C(CH₃)₃ or—C(CD₃)₃. As an example, R² is —C(CD₃)₃. In one aspect of thisembodiment, Y^(1a) and Y^(1b) are hydrogen. In another aspect, Y^(1a)and Y^(1b) are deuterium. In another aspect, Y² is hydrogen. In anotheraspect, Y² is deuterium.

Another embodiment provides compounds wherein Y^(1a) and Y^(1b) are thesame. In one aspect of this embodiment, Y^(1a) and Y^(1b) are deuterium.In one aspect of this embodiment, R² is —C(CD₃)₃.

In yet another embodiment, the compound is selected from any one of thecompounds set forth in Table 1 below.

TABLE 1 Examples of Compounds of Formula I Compound R¹ R² Y^(1a) Y^(1b)Y² 101 CD₃ —C(CH₃)₃ D D H 102 CH₃ —C(CD₃)₃ D D H 103 CH₃ —C(CD₃)₃ H H H104 CD₃ —C(CD₃)₃ H H H 105 CD₃ —C(CD₃)₃ D D H 106 CD₃ —C(CH₃)₃ H H H 107CH₃ —C(CH₃)₃ D D H 108 CD₃ —C(CH₃)₃ D D D 109 CH₃ —C(CD₃)₃ D D D 110 CH₃—C(CD₃)₃ H H D 111 CD₃ —C(CD₃)₃ H H D 112 CD₃ —C(CD₃)₃ D D D 113 CD₃—C(CH₃)₃ H H D 114 CH₃ —C(CH₃)₃ D D D

In certain embodiments, the compound is selected from any one ofCompounds 102, 103, 104, 105, 109, 110, 111 and 112, or pharmaceuticallyacceptable salts thereof. In other embodiments, the compound is selectedfrom any one of Compounds 102, 103 and 105, or pharmaceuticallyacceptable salts thereof. In one aspect, the compound is selected fromCompound 103 or Compound 105, or pharmaceutically acceptable saltsthereof.

In another set of embodiments, any atom not designated as deuterium inany of the embodiments set forth above is present at its naturalisotopic abundance.

The synthesis of compounds of Formula I can be readily achieved bysynthetic chemists of ordinary skill following the Exemplary Synthesisand Examples disclosed herein. Other relevant procedures andintermediates are disclosed, for instance in PCT patent publication Nos.WO 98/04559 and WO 00/44752.

Such methods can be carried out utilizing corresponding deuterated andoptionally, other isotope-containing reagents and/or intermediates tosynthesize the compounds delineated herein, or invoking standardsynthetic protocols known in the art for introducing isotopic atoms to achemical structure. Certain intermediates can be used with or withoutpurification (e.g., filtration, distillation, sublimation,crystallization, trituration, solid phase extraction, andchromatography).

Exemplary Synthesis

Compounds of Formula I may be prepared according to the schemes shownbelow.

Scheme 1. General Route to Compounds of Formula I

The synthesis of compounds of Formula I can be accomplished generally asshown in Scheme 1. Intermediate 12 is prepared by radical alkylation of3,6-dichloropyridazine 11 with the appropriately deuterated pivalic acid10. D9-Pivalic acid is commercially available for the preparation ofthose compounds wherein R² is —C(CD₃)₃. The appropriately deuterated3,6-dichloro-4-t-butylpyridazine 12 is then condensed with2,5-difluorobenzohydrazide 13 to provide 14. Displacement of thechloride with the anion generated from appropriately deuterated(2-methyl-2H-1,2,4-triazol-3-yl)methanol 15 and NaH provides compoundsof Formula I. Alternatively, the conversion of 14 to compounds ofFormula I is accomplished via the use of n-BuLi in THF, or cesiumcarbonate in DMSO, or under other similar conditions known to theskilled artisan.

Scheme 2. Synthesis of Compound 15.

Scheme 2 illustrates the preparation of deuterated analogs of 15. Asdescribed by Dallacker F et al, Chemiker-Zeitung 1986, 110:101-108 andDallacker F et al, Chemiker-Zeitung 1986, 110, p. 275-281,1,2,4-triazole (16) reacts with R¹—I to provide the appropriatelydeuterated methyl triazole 17, which is then treated with formaldehydeor deuterated formaldehyde to provide 15. One skilled in the art willappreciate that deuterium exchange may potentially occur under theseconditions to afford compounds wherein Y² is deuterium.

The specific approaches and compounds shown above are not intended to belimiting. The chemical structures in the schemes herein depict variablesthat are hereby defined commensurately with chemical group definitions(moieties, atoms, etc.) of the corresponding position in the compoundformulae herein, whether identified by the same variable name (i.e., R¹,R², R³, etc.) or not. The suitability of a chemical group in a compoundstructure for use in the synthesis of another compound is within theknowledge of one of ordinary skill in the art.

Additional methods of synthesizing compounds of Formula I and theirsynthetic precursors, including those within routes not explicitly shownin schemes herein, are within the means of chemists of ordinary skill inthe art. Synthetic chemistry transformations and protecting groupmethodologies (protection and deprotection) useful in synthesizing theapplicable compounds are known in the art and include, for example,those described in Larock R, Comprehensive Organic Transformations, VCHPublishers (1989); Greene T W et al., Protective Groups in OrganicSynthesis, 3^(rd) Ed., John Wiley and Sons (1999); Fieser L et al.,Fieser and Fieser's Reagents for Organic Synthesis, John Wiley and Sons(1994); and Paquette L, ed., Encyclopedia of Reagents for OrganicSynthesis, John Wiley and Sons (1995) and subsequent editions thereof.

Combinations of substituents and variables envisioned by this inventionare only those that result in the formation of stable compounds.

Compositions

The invention also provides pyrogen-free compositions comprising aneffective amount of a compound of Formula I (e.g., including any of theformulae herein), or a pharmaceutically acceptable salt-of saidcompound; and an acceptable carrier. Preferably, a composition of thisinvention is formulated for pharmaceutical use (“a pharmaceuticalcomposition”), wherein the carrier is a pharmaceutically acceptablecarrier. The carrier(s) are “acceptable” in the sense of beingcompatible with the other ingredients of the formulation and, in thecase of a pharmaceutically acceptable carrier, not deleterious to therecipient thereof in an amount used in the medicament.

Pharmaceutically acceptable carriers, adjuvants and vehicles that may beused in the pharmaceutical compositions of this invention include, butare not limited to, ion exchangers, alumina, aluminum stearate,lecithin, serum proteins, such as human serum albumin, buffer substancessuch as phosphates, glycine, sorbic acid, potassium sorbate, partialglyceride mixtures of saturated vegetable fatty acids, water, salts orelectrolytes, such as protamine sulfate, disodium hydrogen phosphate,potassium hydrogen phosphate, sodium chloride, zinc salts, colloidalsilica, magnesium trisilicate, polyvinyl pyrrolidone, cellulose-basedsubstances, polyethylene glycol, sodium carboxymethylcellulose,polyacrylates, waxes, polyethylene-polyoxypropylene-block polymers,polyethylene glycol and wool fat.

If required, the solubility and bioavailability of the compounds of thepresent invention in pharmaceutical compositions may be enhanced bymethods well-known in the art. One method includes the use of lipidexcipients in the formulation. See “Oral Lipid-Based Formulations:Enhancing the Bioavailability of Poorly Water-Soluble Drugs (Drugs andthe Pharmaceutical Sciences),” David J. Hauss, ed. Informa Healthcare,2007; and “Role of Lipid Excipients in Modifying Oral and ParenteralDrug Delivery: Basic Principles and Biological Examples,” Kishor M.Wasan, ed. Wiley-Interscience, 2006.

Another known method of enhancing bioavailability is the use of anamorphous form of a compound of this invention optionally formulatedwith a poloxamer, such as LUTROL™ and PLURONIC™ (BASF Corporation), orblock copolymers of ethylene oxide and propylene oxide. See U.S. Pat.No. 7,014,866; and United States patent publications 20060094744 and20060079502.

The pharmaceutical compositions of the invention include those suitablefor oral, rectal, nasal, topical (including buccal and sublingual),vaginal or parenteral (including subcutaneous, intramuscular,intravenous and intradermal) administration. In certain embodiments, thecompound of the formulae herein is administered transdermally (e.g.,using a transdermal patch or iontophoretic techniques). Otherformulations may conveniently be presented in unit dosage form, e.g.,tablets, sustained release capsules, and in liposomes, and may beprepared by any methods well known in the art of pharmacy. See, forexample, Remington: The Science and Practice of Pharmacy, LippincottWilliams & Wilkins, Baltimore, Md. (20th ed. 2000).

Such preparative methods include the step of bringing into associationwith the molecule to be administered ingredients such as the carrierthat constitutes one or more accessory ingredients. In general, thecompositions are prepared by uniformly and intimately bringing intoassociation the active ingredients with liquid carriers, liposomes orfinely divided solid carriers, or both, and then, if necessary, shapingthe product.

In certain embodiments, the compound is administered orally.Compositions of the present invention suitable for oral administrationmay be presented as discrete units such as capsules, sachets, or tabletseach containing a predetermined amount of the active ingredient; apowder or granules; a solution or a suspension in an aqueous liquid or anon-aqueous liquid; an oil-in-water liquid emulsion; a water-in-oilliquid emulsion; packed in liposomes; or as a bolus, etc. Soft gelatincapsules can be useful for containing such suspensions, which maybeneficially increase the rate of compound absorption.

In the case of tablets for oral use, carriers that are commonly usedinclude lactose and corn starch. Lubricating agents, such as magnesiumstearate, are also typically added. For oral administration in a capsuleform, useful diluents include lactose and dried cornstarch. When aqueoussuspensions are administered orally, the active ingredient is combinedwith emulsifying and suspending agents. If desired, certain sweeteningand/or flavoring and/or coloring agents may be added.

Compositions suitable for oral administration include lozengescomprising the ingredients in a flavored basis, usually sucrose andacacia or tragacanth; and pastilles comprising the active ingredient inan inert basis such as gelatin and glycerin, or sucrose and acacia.

Compositions suitable for parenteral administration include aqueous andnon-aqueous sterile injection solutions which may contain anti-oxidants,buffers, bacteriostats and solutes which render the formulation isotonicwith the blood of the intended recipient; and aqueous and non-aqueoussterile suspensions which may include suspending agents and thickeningagents. The formulations may be presented in unit-dose or multi-dosecontainers, for example, sealed ampules and vials, and may be stored ina freeze dried (lyophilized) condition requiring only the addition ofthe sterile liquid carrier, for example water for injections,immediately prior to use. Extemporaneous injection solutions andsuspensions may be prepared from sterile powders, granules and tablets.

Such injection solutions may be in the form, for example, of a sterileinjectable aqueous or oleaginous suspension. This suspension may beformulated according to techniques known in the art using suitabledispersing or wetting agents (such as, for example, Tween 80) andsuspending agents. The sterile injectable preparation may also be asterile injectable solution or suspension in a non-toxicparenterally-acceptable diluent or solvent, for example, as a solutionin 1,3-butanediol. Among the acceptable vehicles and solvents that maybe employed are mannitol, water, Ringer's solution and isotonic sodiumchloride solution. In addition, sterile, fixed oils are conventionallyemployed as a solvent or suspending medium. For this purpose, any blandfixed oil may be employed including synthetic mono- or diglycerides.Fatty acids, such as oleic acid and its glyceride derivatives are usefulin the preparation of injectables, as are naturalpharmaceutically-acceptable oils, such as olive oil or castor oil,especially in their polyoxyethylated versions. These oil solutions orsuspensions may also contain a long-chain alcohol diluent or dispersant.

The pharmaceutical compositions of this invention may be administered inthe form of suppositories for rectal administration. These compositionscan be prepared by mixing a compound of this invention with a suitablenon-irritating excipient which is solid at room temperature but liquidat the rectal temperature and therefore will melt in the rectum torelease the active components. Such materials include, but are notlimited to, cocoa butter, beeswax and polyethylene glycols.

The pharmaceutical compositions of this invention may be administered bynasal aerosol or inhalation. Such compositions are prepared according totechniques well-known in the art of pharmaceutical formulation and maybe prepared as solutions in saline, employing benzyl alcohol or othersuitable preservatives, absorption promoters to enhance bioavailability,fluorocarbons, and/or other solubilizing or dispersing agents known inthe art. See, e.g.: Rabinowitz J D and Zaffaroni A C, U.S. Pat. No.6,803,031, assigned to Alexza Molecular Delivery Corporation.

Topical administration of the pharmaceutical compositions of thisinvention is especially useful when the desired treatment involves areasor organs readily accessible by topical application. For topicalapplication topically to the skin, the pharmaceutical composition shouldbe formulated with a suitable ointment containing the active componentssuspended or dissolved in a carrier. Carriers for topical administrationof the compounds of this invention include, but are not limited to,mineral oil, liquid petroleum, white petroleum, propylene glycol,polyoxyethylene polyoxypropylene compound, emulsifying wax, and water.Alternatively, the pharmaceutical composition can be formulated with asuitable lotion or cream containing the active compound suspended ordissolved in a carrier. Suitable carriers include, but are not limitedto, mineral oil, sorbitan monostearate, polysorbate 60, cetyl esterswax, cetearyl alcohol, 2-octyldodecanol, benzyl alcohol, and water. Thepharmaceutical compositions of this invention may also be topicallyapplied to the lower intestinal tract by rectal suppository formulationor in a suitable enema formulation. Topically-transdermal patches andiontophoretic administration are also included in this invention.

Application of the subject therapeutics may be local, so as to beadministered at the site of interest. Various techniques can be used forproviding the subject compositions at the site of interest, such asinjection, use of catheters, trocars, projectiles, pluronic gel, stents,sustained drug release polymers or other device which provides forinternal access.

Thus, according to yet another embodiment, the compounds of thisinvention may be incorporated into compositions for coating animplantable medical device, such as prostheses, artificial valves,vascular grafts, stents, or catheters. Suitable coatings and the generalpreparation of coated implantable devices are known in the art and areexemplified in U.S. Pat. Nos. 6,099,562; 5,886,026; and 5,304,121. Thecoatings are typically biocompatible polymeric materials such as ahydrogel polymer, polymethyldisiloxane, polycaprolactone, polyethyleneglycol, polylactic acid, ethylene vinyl acetate, and mixtures thereof.The coatings may optionally be further covered by a suitable topcoat offluorosilicone, polysaccharides, polyethylene glycol, phospholipids orcombinations thereof to impart controlled release characteristics in thecomposition. Coatings for invasive devices are to be included within thedefinition of pharmaceutically acceptable carrier, adjuvant or vehicle,as those terms are used herein.

According to another embodiment, the invention provides a method ofcoating an implantable medical device comprising the step of contactingsaid device with the coating composition described above. It will beobvious to those skilled in the art that the coating of the device willoccur prior to implantation into a mammal.

According to another embodiment, the invention provides a method ofimpregnating an implantable drug release device comprising the step ofcontacting said drug release device with a compound or composition ofthis invention. Implantable drug release devices include, but are notlimited to, biodegradable polymer capsules or bullets, non-degradable,diffusible polymer capsules and biodegradable polymer wafers.

According to another embodiment, the invention provides an implantablemedical device coated with a compound or a composition comprising acompound of this invention, such that said compound is therapeuticallyactive.

According to another embodiment, the invention provides an implantabledrug release device impregnated with or containing a compound or acomposition comprising a compound of this invention, such that saidcompound is released from said device and is therapeutically active.

Where an organ or tissue is accessible because of removal from thepatient, such organ or tissue may be bathed in a medium containing acomposition of this invention, a composition of this invention may bepainted onto the organ, or a composition of this invention may beapplied in any other convenient way.

In another embodiment, a composition of this invention further comprisesa second therapeutic agent. The second therapeutic agent may be selectedfrom any compound or therapeutic agent known to have or thatdemonstrates advantageous properties when administered with a compoundhaving the same mechanism of action as L-838417.

Preferably, the second therapeutic agent is an agent useful in thetreatment or prevention of a disease or condition selected fromdisorders of the central nervous system, including anxiety andconvulsions; and neuropathic, inflammatory and migraine associated pain.

In another embodiment, the invention provides separate dosage forms of acompound of this invention and one or more of any of the above-describedsecond therapeutic agents, wherein the compound and second therapeuticagent are associated with one another. The term “associated with oneanother” as used herein means that the separate dosage forms arepackaged together or otherwise attached to one another such that it isreadily apparent that the separate dosage forms are intended to be soldand administered together (within less than 24 hours of one another,consecutively or simultaneously).

In the pharmaceutical compositions of the invention, the compound of thepresent invention is present in an effective amount. As used herein, theterm “effective amount” refers to an amount which, when administered ina proper dosing regimen, is sufficient to reduce or ameliorate theseverity, duration or progression of the disorder being treated, preventthe advancement of the disorder being treated, cause the regression ofthe disorder being treated, or enhance or improve the prophylactic ortherapeutic effect(s) of another therapy.

The interrelationship of dosages for animals and humans (based onmilligrams per meter squared of body surface) is described in Freireichet al., (1966) Cancer Chemother. Rep 50: 219. Body surface area may beapproximately determined from height and weight of the patient. See,e.g., Scientific Tables, Geigy Pharmaceuticals, Ardsley, N.Y., 1970,537.

In one embodiment, an effective amount of a compound of this inventioncan range from about 0.01 to about 5000 mg per treatment. In morespecific embodiments the range is from about 0.1 to 2500 mg, or from 0.2to 1000 mg, or most specifically from about 1 to 500 mg. Treatmenttypically is administered one to three times daily.

Effective doses will also vary, as recognized by those skilled in theart, depending on the diseases treated, the severity of the disease, theroute of administration, the sex, age and general health condition ofthe patient, excipient usage, the possibility of co-usage with othertherapeutic treatments such as use of other agents and the judgment ofthe treating physician. For example, guidance for selecting an effectivedose can be determined by reference to the prescribing information forL-838417.

For pharmaceutical compositions that comprise a second therapeuticagent, an effective amount of the second therapeutic agent is betweenabout 20% and 100% of the dosage normally utilized in a monotherapyregime using just that agent. Preferably, an effective amount is betweenabout 70% and 100% of the normal monotherapeutic dose. The normalmonotherapeutic dosages of these second therapeutic agents are wellknown in the art. See, e.g., Wells et al., eds., PharmacotherapyHandbook, 2nd Edition, Appleton and Lange, Stamford, Conn. (2000); PDRPharmacopoeia, Tarascon Pocket Pharmacopoeia 2000, Deluxe Edition,Tarascon Publishing, Loma Linda, Calif. (2000), each of which referencesare incorporated herein by reference in their entirety.

It is expected that some of the second therapeutic agents referencedabove will act synergistically with the compounds of this invention.When this occurs, it will allow the effective dosage of the secondtherapeutic agent and/or the compound of this invention to be reducedfrom that required in a monotherapy. This has the advantage ofminimizing toxic side effects of either the second therapeutic agent ofa compound of this invention, synergistic improvements in efficacy,improved ease of administration or use and/or reduced overall expense ofcompound preparation or formulation.

Methods of Treatment

In another embodiment, the invention provides a method of inhibiting theα-1 subtype of the GABA-A receptor in a cell, comprising contacting acell with one or more compounds of Formula I herein. In anotherembodiment, the invention provides a method of activating one or more ofthe α2, α3 and α5 subtypes of the GABA-A receptor in a cell.

According to another embodiment, the invention provides a method oftreating a patient suffering from, or susceptible to, a disease that isbeneficially treated by L-838417 comprising the step of administering tosaid patient an effective amount of a compound of Formula I or saltthereof, or a composition of this invention. Such diseases are wellknown in the art and are disclosed in, but not limited to the followingpatents and published applications: WO 1998004559, WO 2000044752, WO2006061428. Such diseases include, but are not limited to, disorders ofthe central nervous system, including anxiety and convulsions; andneuropathic, inflammatory and migraine-associated pain.

Methods delineated herein also include those wherein the patient isidentified as in need of a particular stated treatment. Identifying apatient in need of such treatment can be in the judgment of a patient ora health care professional and can be subjective (e.g. opinion) orobjective (e.g. measurable by a test or diagnostic method).

In another embodiment, any of the above methods of treatment comprisesthe further step of co-administering to said patient one or more secondtherapeutic agents. The choice of second therapeutic agent may be madefrom any second therapeutic agent known to be useful forco-administration with L-838417. The choice of second therapeutic agentis also dependent upon the particular disease or condition to betreated. Examples of second therapeutic agents that may be employed inthe methods of this invention are those set forth above for use incombination compositions comprising a compound of this invention and asecond therapeutic agent.

The term “co-administered” as used herein means that the secondtherapeutic agent may be administered together with a compound of thisinvention as part of a single dosage form (such as a composition of thisinvention comprising a compound of the invention and an secondtherapeutic agent as described above) or as separate, multiple dosageforms. Alternatively, the additional agent may be administered prior to,consecutively with, or following the administration of a compound ofthis invention. In such combination therapy treatment, both thecompounds of this invention and the second therapeutic agent(s) areadministered by conventional methods. The administration of acomposition of this invention, comprising both a compound of theinvention and a second therapeutic agent, to a patient does not precludethe separate administration of that same therapeutic agent, any othersecond therapeutic agent or any compound of this invention to saidpatient at another time during a course of treatment.

Effective amounts of these second therapeutic agents are well known tothose skilled in the art and guidance for dosing may be found in patentsand published patent applications referenced herein, as well as in Wellset al., eds., Pharmacotherapy Handbook, 2nd Edition, Appleton and Lange,Stamford, Conn. (2000); PDR Pharmacopoeia, Tarascon Pocket Pharmacopoeia2000, Deluxe Edition, Tarascon Publishing, Loma Linda, Calif. (2000),and other medical texts. However, it is well within the skilledartisan's purview to determine the second therapeutic agent's optimaleffective-amount range.

In one embodiment of the invention, where a second therapeutic agent isadministered to a subject, the effective amount of the compound of thisinvention is less than its effective amount would be where the secondtherapeutic agent is not administered. In another embodiment, theeffective amount of the second therapeutic agent is less than itseffective amount would be where the compound of this invention is notadministered. In this way, undesired side effects associated with highdoses of either agent may be minimized. Other potential advantages(including without limitation improved dosing regimens and/or reduceddrug cost) will be apparent to those of skill in the art.

In yet another aspect, the invention provides the use of a compound ofFormula I alone or together with one or more of the above-describedsecond therapeutic agents in the manufacture of a medicament, either asa single composition or as separate dosage forms, for treatment orprevention in a patient of a disease, disorder or symptom set forthabove. Another aspect of the invention is a compound of Formula I foruse in the treatment or prevention in a patient of a disease, disorderor symptom thereof delineated herein.

Diagnostic Methods and Kits

The compounds and compositions of this invention are also useful asreagents in methods for determining the concentration of L-838417 insolution or biological sample such as plasma, examining the metabolismof L-838417 and other analytical studies.

According to one embodiment, the invention provides a method ofdetermining the concentration, in a solution or a biological sample, ofL-838417, comprising the steps of:

-   -   a) adding a known concentration of a compound of Formula I to        the solution of biological sample;    -   b) subjecting the solution or biological sample to a measuring        device that distinguishes L-838417 from a compound of Formula I;    -   c) calibrating the measuring device to correlate the detected        quantity of the compound of Formula I with the known        concentration of the compound of Formula I added to the        biological sample or solution; and    -   d) measuring the quantity of L-838417 in the biological sample        with said calibrated measuring device; and    -   e) determining the concentration of L-838417 in the solution of        sample using the correlation between detected quantity and        concentration obtained for a compound of Formula I.

Measuring devices that can distinguish L-838417 from the correspondingcompound of Formula I include any measuring device that can distinguishbetween two compounds that differ from one another only in isotopicabundance. Exemplary measuring devices include a mass spectrometer, NMRspectrometer, or IR spectrometer.

In another embodiment, the invention provides a method of evaluating themetabolic stability of a compound of Formula I comprising the steps ofcontacting the compound of Formula I with a metabolizing enzyme sourcefor a period of time and comparing the amount of the compound of FormulaI with the metabolic products of the compound of Formula I after theperiod of time.

In a related embodiment, the invention provides a method of evaluatingthe metabolic stability of a compound of Formula I in a patientfollowing administration of the compound of Formula I. This methodcomprises the steps of obtaining a serum, urine or feces sample from thepatient at a period of time following the administration of the compoundof Formula I to the subject; and comparing the amount of the compound ofFormula I with the metabolic products of the compound of Formula I inthe serum, urine or feces sample.

The present invention also provides kits for use to treat disorders ofthe central nervous system, including anxiety and convulsions; andneuropathic, inflammatory and migraine associated pain. These kitscomprise (a) a pharmaceutical composition comprising a compound ofFormula I or a salt thereof, wherein said pharmaceutical composition isin a container; and (b) instructions describing a method of using thepharmaceutical composition to treat disorders of the central nervoussystem, including anxiety and convulsions; and neuropathic, inflammatoryand migraine associated pain.

The container may be any vessel or other sealed or sealable apparatusthat can hold said pharmaceutical composition. Examples include bottles,ampules, divided or multi-chambered holders bottles, wherein eachdivision or chamber comprises a single dose of said composition, adivided foil packet wherein each division comprises a single dose ofsaid composition, or a dispenser that dispenses single doses of saidcomposition. The container can be in any conventional shape or form asknown in the art which is made of a pharmaceutically acceptablematerial, for example a paper or cardboard box, a glass or plasticbottle or jar, a re-sealable bag (for example, to hold a “refill” oftablets for placement into a different container), or a blister packwith individual doses for pressing out of the pack according to atherapeutic schedule. The container employed can depend on the exactdosage form involved, for example a conventional cardboard box would notgenerally be used to hold a liquid suspension. It is feasible that morethan one container can be used together in a single package to market asingle dosage form. For example, tablets may be contained in a bottle,which is in turn contained within a box. In one embodiment, thecontainer is a blister pack.

The kits of this invention may also comprise a device to administer orto measure out a unit dose of the pharmaceutical composition. Suchdevice may include an inhaler if said composition is an inhalablecomposition; a syringe and needle if said composition is an injectablecomposition; a syringe, spoon, pump, or a vessel with or without volumemarkings if said composition is an oral liquid composition; or any othermeasuring or delivery device appropriate to the dosage formulation ofthe composition present in the kit.

In certain embodiment, the kits of this invention may comprise in aseparate vessel of container a pharmaceutical composition comprising asecond therapeutic agent, such as one of those listed above for use forco-administration with a compound of this invention.

EXAMPLES Example 1 Synthesis of7-(tert-Butyl-d₉)-3-(2,5-difluorophenyl)-6-((1-methyl-1H-1,2,4-triazol-5-yl)methoxy)-[1,2,4]triazolo[4,3-b]pyridazine(Compound 103)

Compound 103 was prepared from appropriately deuterated intermediates asgenerally outlined in Scheme 1 above.

Step 1. 4-(tert-Butyl-d₉)-3,6-dichloropyridazine (12a)

Concentrated sulfuric acid (5.7 mL, 108 mmol) was added to a suspensionof freshly purified 3,6-dichloro-pyridazine, 11 (5.4 g, 33.5 mmol) indistilled water (130 mL). The mixture was warmed to 65° C. andtrimethylacetic acid-d₉, 10a (6.0 g, 54 mmol, CDN Isotopes, 99 atom % D)was added, followed by silver nitrate (1.1 g, 7 mmol). A solution ofammonium peroxydisulfate (12.3 g, 54 mmol) in distilled water (35 mL)was added to the mixture over a 10-15 minute period while keeping thereaction temperature at 65-75° C. The mixture was stirred for 30 minutesand cooled to room temperature. The mixture was poured onto ice (100 g)and the mixture adjusted to pH=9-10 with concentrated ammoniumhydroxide. The aqueous mixture was extracted with dichloromethane (2×30mL). The combined extracts were washed with 1N sodium hydroxide (10 mL),dried over Na₂SO₄, filtered and concentrated under reduced pressure. Thecrude product was purified by column chromatography on silica gel,eluting with 10% ethyl acetate/heptanes to give 6.1 g (80%) of 12a as acolorless oil.

Step 2.7-(tert-Butyl-d₉)-6-chloro-3-(2,5-difluorophenyl)-[1,2,4]triazolo[4,3-b]pyridazine(14a)

A mixture of 12a (6 g, 28 mmol), 13 (7.2 g, 42 mmol, commerciallyavailable), and triethylamine hydrochloride (5.8 g, 42 mmol) in xylene(30 mL) was heated at 150° C. with stirring for 36 hours. After coolingto room temperature the mixture was concentrated under reduced pressure.The residue was triturated with dichloromethane (40 mL), filtered, andthe filtrate concentrated under reduced pressure. The crude product waspurified by chromatography on silica gel, eluting with 20-50% ethylacetate/heptanes to give 5.6 g (60%) of 14a as an off-white solid.

Step 3.7-(tert-Butyl-d₉)-3-(2,5-difluorophenyl)-6-((1-methyl-1H-1,2,4-triazol-5-yl)methoxy)-[1,2,4]triazolo[4,3-b]pyridazine(Compound 103)

To a solution of (1-methyl-1H-1,2,4-triazol-5-yl)methanol 15a (0.45 g,4.0 mmol, commercially available) in DMF (20 mL) was added 60% sodiumhydride in mineral oil (0.17, 4.3 mmol). The mixture was stirred for 15minutes and 14a (1.2 g, 3.6 mmol) was added. The mixture was stirred for3 hours at room temperature, then diluted with water (100 mL). Theprecipitate was collected by filtration and washed several times withwater. The crude product was purified by column chromatography on silicagel eluting with 5% methanol/dichloromethane. The product was furtherpurified by recrystallization from ethyl acetate-heptanes (1:1) to give1.25 g (78%) of Compound 103 as a white solid. ¹H-NMR (300 MHz, CDCl₃):δ 3.91 (s, 3H), 5.55 (s, 2H), 7.23-7.28 (m, 2H), 7.62-7.68 (m, 1H), 7.93(s, 1H), 8.00 (s, 1H). ¹³C-NMR (75 MHz, CDCl₃): δ 34.55, 35.66, 59.37,115.58 (dd, J₁=16.6, J₂=9.2), 117.63 (dd, J₁=25.8, J₂=6.6), 117.72 (dd,J₁=24.5, J₂=12.2), 118.72 (dd, J₁=24.0, J₂=8.5), 121.74, 137.85, 143.47,145.00, 149.49, 151.13, 155.70 (d, J=160.9), 159.01 (d, J=155.9),158.70. HPLC (method: Waters Atlantis T3 2.1×50 mm 3 μm C18-RPcolumn—gradient method 5-95% ACN+0.1% formic acid in 14 min (1.0 mL/min)with 4 min hold at 95% ACN; Wavelength: 254 nm): retention time: 5.41min; 99.3% purity. MS (M+H): 409.2. Elemental Analysis (C₁₉H₁₀D₉F₂N₇O):Calculated: C=55.88; H=4.69; N=24.01. Found: C=55.98; H=4.53; N=23.98.

Example 2 Synthesis of(1-(Methyl-d₃)-1H-1,2,4-triazol-5-yl)-1,1-d₂-methanol (15b)

Intermediate 15b was prepared from appropriately deuteratedintermediates as generally outlined in Scheme 2 above.

Step 1. 1-(Methyl-d₃)-1H-1,2,4-triazole (17a)

In a flask equipped with a mechanical stirrer under a nitrogenatmosphere, 1,2,4-triazole 16 (6.0 g, 87 mol) was added to anhydrous THF(60 mL) followed by the addition of iodomethane-d₃ (6.5 mL, 1.05 mol,Cambridge Isotopes, 99 atom % D). The cloudy mixture was cooled to 0° C.and 1,8-diazabicyclo[5.4.0]undec-7-ene “DBU” (13.2 mL, 0.87 mol) wasadded over 20 minutes. The mixture was allowed to slowly warm to roomtemperature and was stirred overnight. The mixture was then filteredthrough a Celite pad, and the filtrate concentrated under reducedpressure to give 7.3 g (>100%) of crude 17a as a yellow oil. GCMS showsa purity of 90%. The ratio of regioisomers was 12:1.

Step 2. (1-(Methyl-d₃)-1H-1,2,4-triazol-5-yl)-1,1-d₂-methanol (15b)

A mixture of 17a (5 g, 58 mmol) and paraformaldehyde-d₂ (10 g, 333 mmol,Cambridge Isotopes, 99 atom % D) was heated in a sealed tube at 170° C.for 5 hours. The mixture was cooled to room temperature and diluted withdichloromethane (20 mL). The solid was removed by filtration and thefiltrate was concentrated under reduced pressure. The crude product waspurified by chromatography on a short column of silica gel, eluting with75% THF/heptanes to give 4.8 g (71%) of 15b as an off-white solid.

Example 3 Synthesis of7-(tert-Butyl-d₉)-3-(2,5-difluorophenyl)-6-((1-(methyl-d₃)-1H-1,2,4-triazol-5-yl)-1,1-d₂-methoxy)-[1,2,4]triazolo[4,3-b]pyridazine(Compound 105)

Compound 105 was prepared from appropriately deuterated intermediates asgenerally outlined in Scheme 1 above.

7-(tert-Butyl-d₉)-3-(2,5-difluorophenyl)-6-((1-(methyl-d₃)-1H-1,2,4-triazol-5-yl)-1,1-d₂-methoxy)-[1,2,4]triazolo[4,3-b]pyridazine(Compound 105)

To a solution of 15b (0.24 g, 2.0 mmol) in DMF (20 mL) was added 60%sodium hydride in mineral oil (0.08, 2.1 mmol). The mixture was stirredfor 15 minutes and 14a (0.6 g, 1.8 mmol, see Example 1) was added. Themixture was stirred for 3 hours at room temperature, then diluted withwater (100 mL). The precipitate was collected by filtration and washedseveral times with water. The crude product was purified by columnchromatography on silica gel eluting with 5% methanol/dichloromethane.The product was further purified by recrystallization from ethylacetate/heptane (1:1) to give 0.52 g (70%) of Compound 105 as a whitesolid. ¹H-NMR (300 MHz, CDCl₃): δ 7.23-7.28 (m, 2H), 7.63-7.67 (m, 1H),7.92 (s, 1H), 8.00 (s, 1H). ¹³C-NMR (75 MHz, CDCl₃): absence of signalsat 35.66 and 59.37. HPLC (method: Waters Atlantis T3 2.1×50 mm 3 μmC18-RP column—gradient method 5-95% ACN+0.1% formic acid in 14 min (1.0mL/min) with 4 min hold at 95% ACN; Wavelength: 254 nm): retention time:5.40 min; 99.0% purity. MS (M+H): 414.3. Elemental Analysis(C₁₉H₅D₁₄F₂N₇O): Calculated: C=55.20; H=4.63; N=23.72; F=9.19. Found:C=54.88; H=4.45; N=23.46; F=9.59.

Example 4 Synthesis of (1-(Methyl-d₃)-1H-1,2,4-triazol-5-yl)-methanol(15c)

Intermediate 15c was prepared from appropriately deuteratedintermediates as generally outlined in Scheme 2 above.

(1-(Methyl-d₃)-1H-1,2,4-triazol-5-yl)-methanol (15c)

A mixture of 17a (5 g, 58 mmol, see Example 2) and paraformaldehyde (10g, 333 mmol) was heated in a sealed tube at 170° C. for 5 hours. Themixture was cooled to room temperature and diluted with dichloromethane(20 mL). The solid was removed by filtration and the filtrate wasconcentrated under reduced pressure. The crude product was purified bychromatography on a short column of silica gel, eluting with 5%methanol/dichloromethane to give 5.0 g (75%) of 15c as an off-whitesolid.

Example 5 Synthesis of7-(tert-Butyl-d₉)-3-(2,5-difluorophenyl)-6-((1-(methyl-d₃)-1H-1,2,4-triazol-5-yl)-methoxy)-[1,2,4]triazolo[4,3-b]pyridazine(Compound 104)

Compound 104 was prepared from appropriately deuterated intermediates asgenerally outlined in Scheme 1 above.

7-(tert-Butyl-d₉)-3-(2,5-difluorophenyl)-6-((1-(methyl-d₃)-1H-1,2,4-triazol-5-yl)-methoxy)-[1,2,4]triazolo[4,3-b]pyridazine(Compound 104)

To a solution of 15c (0.46 g, 4.0 mmol) in DMF (20 mL) was added 60%sodium hydride in mineral oil (0.17, 4.3 mmol). The mixture was stirredfor 15 minutes and 14a (1.2 g, 3.6 mmol, see Example 1) was added. Themixture was stirred for 3 hours at room temperature, then diluted withwater (100 mL). The precipitate was collected by filtration and washedseveral times with water. The crude product was purified by columnchromatography on silica gel eluting with 5% methanol/dichloromethane.The product was further purified by recrystallization from ethylacetate/heptane (1:1) to give 1.31 g (88%) of Compound 104 as a whitesolid. ¹H-NMR (300 MHz, CDCl₃): δ 5.55 (s, 2H), 7.23-7.28 (m, 2H),7.62-7.67 (m, 1H), 7.93 (s, 1H), 8.00 (s, 1H). ¹³C-NMR (75 MHz, CDCl₃):δ 34.55, 59.36, 115.53 (dd, J₁=16.6, J₂=8.8), 117.63 (dd, J₁=24.4,J₂=12.8), 117.71 (dd, J₁=24.1, J₂=8.0), 118.77 (dd, J₁=23.9, J₂=8.5),121.75, 137.85, 143.48, 145.00, 149.50, 151.15, 155.76 (d, J=163.5),159.08 (d, J=156.9), 158.71. HPLC (method: Waters Atlantis T3 2.1×50 mm3 μm C18-RP column—gradient method 5-95% ACN+0.1% formic acid in 14 min(1.0 mL/min) with 4 min hold at 95% ACN; Wavelength: 254 nm): retentiontime: 5.40 min; 99.6% purity. MS (M+H): 412.2. Elemental Analysis(C₁₉H₇D₁₂F₂N₇O): Calculated: C=55.47; H=4.67; N=23.83. Found: C=55.49;H=4.76; N=23.87.

Example 6 Synthesis of7-tert-Butyl-3-(2,5-difluorophenyl)-6-((1-(methyl-d₃)-1H-1,2,4-triazol-5-yl)-1,1-d₂-methoxy)-[1,2,4]triazolo[4,3-b]pyridazine(Compound 101)

Compound 101 was prepared from appropriately deuterated intermediates asgenerally outlined in Scheme 1 above.

7-tert-Butyl-3-(2,5-difluorophenyl)-6-((1-(methyl-d₃)-1H-1,2,4-triazol-5-yl)-1,1-d₂-methoxy)-[1,2,4]triazolo[4,3-b]pyridazine(Compound 101)

To a solution of 15b (0.24 g, 4.0 mmol, see Example 2) in DMF (10 mL)was added 60% sodium hydride in mineral oil (0.08, 2.1 mmol). Themixture was stirred for 15 minutes and known compound7-tert-butyl-6-chloro-3-(2,5-difluorophenyl)-[1,2,4]triazolo[4,3-b]pyridazine,14b (0.58 g, 1.8 mmol, prepared as described in patent applicationWO1998004559) was added. The mixture was stirred for 3 hours at roomtemperature, then diluted with water (100 mL). The precipitate wascollected by filtration and washed several times with water. The crudeproduct was purified by column chromatography on silica gel eluting with75% THF/heptanes. The product was further purified by recrystallizationfrom ethyl acetate/heptane (1:1) to give 0.53 g (72%) of Compound 101 asa white solid. ¹H-NMR (300 MHz, CDCl₃): δ 1.41 (s, 9H), 7.23-7.28 (m,2H), 7.62-7.68 (m, 1H), 7.92 (s, 1H), 8.00 (s, 1H). ¹³C-NMR (75 MHz,CDCl₃): appearance of signal at 28.96, absence of signal at 35.66 and59.36. HPLC (method: Waters Atlantis T3 2.1×50 mm 3 μm C18-RPcolumn—gradient method 5-95% ACN+0.1% formic acid in 14 min (1.0 mL/min)with 4 min hold at 95% ACN; Wavelength: 254 nm): retention time: 5.42min; 99.7% purity. MS (M+H): 405.3. Elemental Analysis (C₁₉H₁₄D₅F₂N₇O):Calculated: C=56.43; H=4.74; N=24.25; F=9.40. Found: C=56.22; H=4.73;N=23.87; F=9.35.

Example 7 Evaluation of Metabolic Stability

Human liver microsomes (20 mg/mL) and rat liver microsomes (20 mg/mL)were obtained from Xenotech, LLC (Lenexa, Kans.). β-nicotinamide adeninedinucleotide phosphate, reduced form (NADPH), magnesium chloride(MgCl₂), and dimethyl sulfoxide (DMSO) were purchased fromSigma-Aldrich.

Determination of Metabolic Stability:

7.5 mM stock solutions of test compounds (L-838417, Compound 101,Compound 103, Compound 104 and Compound 105) were prepared in DMSO. The7.5 mM stock solutions were diluted to 12.5 μM in acetonitrile (ACN).The 20 mg/mL liver microsomes (either human or liver) were diluted to2.5 mg/mL in 0.1 M potassium phosphate buffer, pH 7.4, containing 3 mMMgCl₂. The diluted microsomes (375 μL) were added to wells of a 96-welldeep-well polypropylene plate in triplicate. 10 μL of the 12.5 μM testcompound was added to the microsomes and the mixture was pre-warmed for10 minutes. Reactions were initiated by addition of 125 μL pre-warmedNADPH solution. The final reaction volume is 0.5 mL and contained 0.5mg/mL human liver microsomes, 0.25 μM test compound, and 2 mM NADPH in0.1 M potassium phosphate buffer, pH 7.4, and 3 mM MgCl₂. The reactionmixtures are incubated at 37° C., and 50 μL aliquots were removed at 0,5, 10, 20, and 30 minutes and added to shallow-well 96-well plates whichcontained 50 μL of ice-cold ACN with internal standard to stop thereactions. The plates were stored at 4° C. for 20 minutes after which100 μL of water was added to the wells of the plate beforecentrifugation to pellet precipitated proteins. Supernatants weretransferred to another 96-well plate and analyzed for amounts of parentremaining by LC-MS/MS using an Applied Bio-systems API 4000 massspectrometer. 7-ethoxycoumarin (1 μM) was used as a positive control.

Data Analysis:

The in vitro t_(1/2)s for test compounds were calculated from the slopesof the linear regression of % parent remaining (ln) vs incubation timerelationship, using the formula:

in vitro t _(1/2)=0.693/k, where k=−[slope of linear regression of %parent remaining(ln) vs incubation time]

Data analysis is performed using Microsoft Excel Software.

The results of these experiments are shown in FIGS. 1 (human livermicrosomes) and 2 (rat liver microsomes). As shown in FIG. 1,approximately 70% of L-838417 remained intact after 30 minutes ofincubation with human liver microsomes. The half life of L-838417 wascalculated to be 54.4 minutes. In contrast, each of Compounds 101, 103,104 and 105 were stable in human liver microsomes (more than 80% ofparent compound remained intact after 30 minute incubation).

In rat liver microsomes, approximately 70% of both L-838417 and Compound101 remained intact after 30 minutes of incubation (FIG. 2). Thecalculated half-lives were 50.5 minutes for L-838417 and 52.7 minutesfor Compound 101. More than 80% of each of Compounds 103, 104 and 105remained intact after 30 minutes of incubation, and each was consideredstable in rat microsomes.

Example 8 Pharmacokinetic and Bioavailability Analysis of Compounds 103and 105 Following Oral and Intravenous Administration to Rats

Three male Sprague-Dawley rats (200-250 g each) were cannulated in thejugular vein and were administered a single dose containing 1 mg/kg eachof L-838417, Compound 103 and Compound 105 (as a 1:1:1 mixturecontaining 1 mg/mL of each of the three compounds in 10% dimethylsulfoxide (DMSO), 10% N,N-dimethylacetamide (DMA), and 60% polyethyleneglycol (PG) through the jugular cannula. Three additional maleSprague-Dawley rats (200-250 g each) were administered a single dosecontaining 1 mg/kg each of L-838417, Compound 103 and Compound 105 (as a1:1:1 mixture containing 1 mg/mL of each of the three compounds in 10%dimethyl sulfoxide (DMSO), 10% N,N-dimethylacetamide (DMA), and 60%polyethylene glycol (PG) by oral gavage.

Blood (0.25 mL) from intravenously treated test rats was collectedretro-orbitally at 2, 5, 15, and 30 minutes, and 1, 2, 4, and 6 hourspost-dosing. Blood (0.25 mL) from orally treated test rats was collectedretro-orbitally at 5, 15, 30, and 45 minutes, and 1, 2, 4, and 6 hourspost-dosing. Blood was collected into tubes containing K₂EDTA asanticoagulant at the above mentioned time points. Blood samples werestored on ice and then centrifuged to obtain plasma. The plasma (˜0.125μL) was aliquoted into deep 96-well plates and stored at −80° C. untilanalysis by LC-MS/MS using an Applied Bio-systems API 4000 massspectrometer.

The results of the intravenous portion of this study are shown in FIG. 3and in Table 2, below.

TABLE 2 Pharmacokinetics of L-838417, Compound 103 and Compound 105after Intravenous Administration to Sprague-Dawley Rats. % Change over %Change over L-838417 Compound 103 L-838417 Compound 105 L-838417 t_(1/2)(hr) 0.7 ± 0.04 1.25 ± 0.14 +79 1.27 ± 0.14 +81 CL 1.1 ± 0.11 0.48 ±0.05 −62 0.46 ± 0.05 −60 (L/hr/kg) AUC₀₋₆ 831 ± 86  1920 ± 200  +1311957 ± 190  +136 (hr * ng/mL)These results demonstrate that Compounds 103 and 105 each have anapproximately 80% longer half-life and produce a greater than two-foldhigher AUC₀₋₆ than undeuterated L-838417 following intravenousadministration to rats. In addition, both Compound 103 and 105 are eachcleared approximately 60% more slowly than L-838417.

The results of the oral administration portion of this study are shownin FIG. 4 and in Table 3, below.

TABLE 3 Pharmacokinetics of L-838417, Compound 103 and Compound 105after Oral Administration to Sprague-Dawley Rats. % % Change ChangeCompound over L- Compound over L- L-838417 103 838417 105 838417 C_(max)128 ± 40 277 ± 76 +116 280 ± 80 +119 (ng/mL) AUC₀₋₆ 491 ± 95 1258 ± 289+156 1248 ± 272 +154 (hr * ng/mL)These results show that Compounds 103 and 105 demonstrate a greater thantwo-fold higher C_(max) following oral administration in rats thanL-838417. In addition, the results show that Compounds 103 and 105produce a greater than two and a half-fold higher AUC₀₋₆ thanundeuterated L-838417 following oral administration.

Without further description, it is believed that one of ordinary skillin the art can, using the preceding description and the illustrativeexamples, make and utilize the compounds of the present invention andpractice the claimed methods. It should be understood that the foregoingdiscussion and examples merely present a detailed description of certainpreferred embodiments. It will be apparent to those of ordinary skill inthe art that various modifications and equivalents can be made withoutdeparting from the spirit and scope of the invention. All the patents,journal articles and other documents discussed or cited above are hereinincorporated by reference.

1. A compound of Formula I:

or a pharmaceutically acceptable salt thereof, wherein: R¹ is CH₃, CDH₂,CD₂H, or CD₃; R² is a t-butyl group having 0-9 deuterium atoms; each Yis independently hydrogen or deuterium; and when R¹ is CH₃ and each Y ishydrogen, then R² has 1-9 deuterium.
 2. The compound of claim 1, whereinR¹ is CH₃ or CD₃.
 3. The compound of claim 1, wherein R² is —C(CH₃)₃ or—C(CD₃)₃.
 4. The compound of claim 1, wherein Y^(1a) and Y^(1b) are thesame.
 5. The compound of claim 1, wherein R² is —C(CD₃)₃.
 6. Thecompound of claim 1, selected from any one of the compounds set forth intable below: Compound R¹ R² Y^(1a) Y^(1b) Y² 101 CD₃ —C(CH₃)₃ D D H 102CH₃ —C(CD₃)₃ D D H 103 CH₃ —C(CD₃)₃ H H H 104 CD₃ —C(CD₃)₃ H H H 105 CD₃—C(CD₃)₃ D D H 106 CD₃ —C(CH₃)₃ H H H 107 CH₃ —C(CH₃)₃ D D H 108 CD₃—C(CH₃)₃ D D D 109 CH₃ —C(CD₃)₃ D D D 110 CH₃ —C(CD₃)₃ H H D 111 CD₃—C(CD₃)₃ H H D 112 CD₃ —C(CD₃)₃ D D D 113 CD₃ —C(CH₃)₃ H H D 114 CH₃—C(CH₃)₃ D D D

or a pharmaceutically acceptable salt thereat.
 7. The compound of claim1, wherein any atom not designated as deuterium in any of theembodiments set forth above is present at its natural isotopicabundance.
 8. A pyrogen-free composition comprising a compound of claim1; and an acceptable carrier.
 9. The composition of claim 8 formulatedfor pharmaceutical administration, wherein the carrier is apharmaceutically acceptable carrier.
 10. The composition of claim 9additionally comprising a second therapeutic agent useful in thetreatment or prevention of a disease or condition selected fromdisorders of the central nervous system, neuropathic pain, inflammatorypain, and migraine-associated pain.
 11. A method of: a. inhibiting theα-1 subtype of the GABA-A receptor in a cell; or b. activating one ormore of the α2, α3 and α5 subtypes of the GABA-A receptor in a cell, themethod comprising contacting the cell with a compound of claim
 1. 12. Amethod of treating a patient suffering from, or susceptible to, adisease or condition selected from disorders of the central nervoussystem, neuropathic pain, inflammatory pain, and migraine-associatedpain, comprising the step of administering to the patient in needthereof a composition of claim
 9. 13. The method of claim 12, whereinthe patient is suffering from or susceptible to anxiety or convulsions.14. The method of claim 12, comprising the additional step ofco-administering to the patient in need thereof a second therapeuticagent useful in the treatment of disorders of the central nervoussystem, neuropathic pain, inflammatory pain, or migraine-associatedpain.